1. Field of the Invention
The present invention relates to direct injection internal combustion engines, and more particularly, to the configuration of an internal combustion engine employing a spray guided combustion method.
2. Description of the Related Art
The spray guided combustion method is known as a combustion method for direct injection internal combustion engines. In a direct injection internal combustion engine using the spray guided combustion method, a spark plug is arranged such that an electrode section thereof is located within or near a fuel injection region where a fuel spray is injected from a fuel injection valve to produce an air-fuel mixture, and the air-fuel mixture thus produced is ignited by the spark plug while passing near the electrode section. With the spray guided combustion method, a relatively rich air-fuel mixture can be formed in the vicinity of the electrode section while the mixture in other regions remote from the electrode section can be made ultra lean, thus enabling what is called stratified lean burn to take place inside a cylinder by means of an air-fuel mixture which is lean as a whole, while at the same time ensuring satisfactory combustion stability, whereby the fuel consumption can be improved.
Generally, the electrode section of the spark plug is constituted by a center electrode and a rod-shaped ground electrode (hereinafter referred to as side electrode) arranged on one side of the center electrode. In the direct injection internal combustion engine using the spray guided combustion method, fuel is injected from one side of the electrode section. Thus, depending on the position of the side electrode, the side electrode may obstruct the passage of the fuel spray, possibly making it difficult for the fuel to reach the electrode section. It is therefore desirable that the spark plug should be fixed in an angular position such that the side electrode does not hinder the supply of the fuel to the electrode section. However, since the spark plug is mounted to a cylinder head of the engine with an external thread thereof screwed into an internal thread cut in the cylinder head, it is difficult to fix the spark plug so that the side electrode may invariably be located at a given rotational angular position. Where the internal combustion engine is a single-cylinder engine, even if the side electrode of the spark plug is not located exactly at the given rotational angular position, acceptable combustion stability can be obtained by appropriately setting the fuel injection timing or the ignition timing in accordance with the actual rotational angular position of the side electrode. In the case of a multi-cylinder engine, on the other hand, the rotational angular position of the side electrode of the spark plug can vary from cylinder to cylinder, possibly making it difficult to set the ignition timing and the injection timing.
To eliminate the inconvenience, a technique facilitating the setting of the ignition timing and injection timing of a multi-cylinder direct fuel injection internal combustion engine has been developed, as disclosed in Japanese Laid-open Patent Publication No. 2006-291798. According to this technique, a cylindrical shroud is provided at the distal end of the spark plug so as to surround the electrode section, and a plurality of fuel spray passage holes are formed in the shroud at regular intervals in a circumferential direction of same so that fuel may be supplied to the electrode section with uniform efficiency regardless of the rotational angular position of the side electrode of the spark plug.
Where the electrode section is surrounded by the cylindrical shroud as disclosed in Japanese Laid-open Patent Publication No. 2006-291798, vaporization of the fuel is accelerated by the shroud, but since the size of the individual fuel spray passage holes that can be formed in the shroud is limited, it cannot be said that the fuel spray is efficiently supplied to the electrode section.